Validate the shape of the cacheMap at the initialization time

[gajus]

The current implementation assumes that the cacheMap object has the required shape without making checks. This results in the DataLoader failing in hard to replace scenarios.

In the specific case I was debugging, I've started getting an error:

TypeError: this._promiseCache.delete is not a function
    at DataLoader.clear (/Users/gajus/Documents/dev/applaudience/showtime-api/node_modules/dataloader/index.js:126:30)
    at /Users/gajus/Documents/dev/applaudience/showtime-api/node_modules/dataloader/index.js:292:12
    at Array.forEach (native)
    at failedDispatch (/Users/gajus/Documents/dev/applaudience/showtime-api/node_modules/dataloader/index.js:288:9)
    at /Users/gajus/Documents/dev/applaudience/showtime-api/node_modules/dataloader/index.js:281:12

This error appeared because I am using lru-cache instead of Map to implement the cache. Turns out that lru-cache API is not compatible with Map and therefore does not work with DataLoader without a wrapper. The problem is that I've only learned this in an error scenario when DataLoader had to use delete strategy.

The solution: validate the shape of the cacheMap input at the time of constructing an instance of DataLoader to assert that it has all the required methods required to work with DataLoader.

[wincent]

Thanks for filing the issue, @gajus.

I think it would be desirable to fix this upstream (like you've proposed), but some runtime validation wouldn't hurt either in order to make the dev experience a little friendlier. I think we could entertain a PR for this one.